Coated materials

ABSTRACT

A thin fluoropolymer film is bonded to a substrate for medical, filtration or packaging purposes by using a pulsed or combination of gas and electrical pulsed cold plasma polymerization procedures.

[0001] This invention relates to a method of applying a fluoropolymerfilm to a body and to bodies so treated.

[0002] Oleophobic or superhydrophobic surfaces are desired for a numberof applications. The invention arises out of investigations of thephenomenon of surfaces with lower energy than ptfe(polytetrafluoroethylene) by taking advantage of the effect arising fromattachment of CF₃ groups to a variety of materials.

[0003] The invention may be applicable to thin films usable in polymericfilter media and to cold plasma treatments to create low energy surfacesupon low-cost thermoplastics and natural media, and to thefunctionalisation of fluorinated polymers such as PTFE and PVDF(polyvinylidene difluoride). This specification discusses a plasmaprocedure leading to a thin film of perfluoroalkyl groups upon asubstrate, which will exhibit superhydrophobicity or oleophobicity. Bythis we mean that the surface will repel liquid with surface energies aslow as that of acetone and alcohol.

[0004] The controlled deposition of many plasma polymers has beenexamined and the ratio of CF₂ to CF₃ is documented in terms of monomertype, plasma power levels and proximity to the glow region. We are nowdescribing a new method for creating surfaces with greater coverage offunctional groups which offers a novel approach to the creation ofpolymer surfaces by pulsed gas introduction to the plasma.

[0005] According to the present invention, a method of applying afluoropolymer film to a porous or microporous or other body, comprisesexposing the body to cold plasma polymerisation using a pulsed gasregime to form either (i) an adherent layer of unsaturated carboxylic(e.g. acrylic) acid polymer on the surface and then derivatising thepolymer to attach a perfluoroalkyl group terminating in—CF₃trifluoromethyl, or (ii) a polymer of a perfluorocarbon monomer. Acombination of electrical and gas pulsing may be used.

[0006] Preferably, the cold method of applying a fluoropolymer filmaccording to 1 and 2 wherein the cold plasma polymerisation uses aperfluorocarbon monomer or an unsaturated carboxylic acid.

[0007] The “gas on” and “gas off” times are preferably from 0.1microsecond to 10 seconds.

[0008] The pulsed gas may be oxygen (except when perfluorocarbon monomeris used), or may be a noble or inert gas or H₂, N₂ or CO₂.Alternatively, acrylic acid polymer precursor or perfluorocarbon monomermay be pulsed directly without a process gas.

[0009] The body may be a film (not necessarily microporous) or of othergeometry that allows coating by plasma polymerisation to a standard ofconsistency adequate for the end use.

[0010] The method may be stopped at any stage, when the applied film iscontinuous and impervious or at an earlier stage, when it is to agreater or lesser extent still apertured, i.e. has not yet completelyfilled in the underlying pores of the body. The pore size of thefinished product can be set to any desired value by ceasing the methodafter an appropriate duration.

[0011] The plasma power is preferably 1W to 100W, more preferably 1.5Wto 7W, except possibly where perfluorocarbon monomer is used.

[0012] The invention extends to the body with the thus-applied film. Thesubstrate material of the body may be carbonaceous (e.g. a naturalmaterial such as cellulose, collagen or alginate, e.g. linen),synthetic, ceramic or metallic or a combination of these.

[0013] Electrical pulsing of the radio frequency supply to the plasma isknown. This technique can ensure a more rapid deposition and greatercoverage of the substrate surface by the plasma polymer. We haveutilised the plasma polymerisation of acrylic acid, which again is knownbut using a pulsed gas regime and clearly there are many other possibleunsaturated carboxylic acids available as monomers. It is believed thatsuch functionalities impart a degree of biocompatibility to substratesand allow of cell culture experiments to be undertaken successfully uponsuch a surface even with difficult and sensitive cell lines.

[0014] By virtue of a derivatisation stage, the acid group may bereacted with a range of materials, for example perfluoralkylamines, toyield a surface rich in perfluoralkylamines groups. In this way thesurface would predominate in CF₃ functions. Additionally the use offluorinated surfactants will similarly generate a surface film of lowerenergy than ptfe and find application in for example the packagingmarket where oleophobic materials are desirable.

[0015] In the packaging market, there is a need for oleophobic ventingfilms where the contents of a vessel or a package may require therelease of a differential pressure. Such pressure differentials mayarise from expansion or contraction of the container or the liquidcontents, with changes in the ambient temperature or pressure. Theliquid contents must be retained without leakage and so porous ventingaids are used. In those situations where liquids of low surface tensionare involved e.g. surfactants, detergents, or organic solvents, thenconventional porous ptfe materials are not as efficient. The surfaceenergy of such materials is of the order of 18 to 20 dynes/cm at 20° C.and the energy of a CF₃ surface is less at perhaps 6 dynes/cm, and canbe influenced by the plasma conditions used for the deposition. It isalso known that the substrate morphology can influence the value of thecontact angle since surfaces of a certain roughness can lead tocomposite angles. The surface which has the greatest number of CF₃groups packed together will have the lowest surface energy.

[0016] Products having superior (high density) surface coverage, rapidlydeposited, may arise from gas pulsing alone or in combination with R.F.pulsing. Such materials have application in filtration, chromatography,medical device and laboratory ware. For example low cost thermoplasticscould be coated using perfluorocarbon monomers to afford ptfe-likeproperties.

[0017] The body or substrate upon which the superhydrophobic layer isattached may be a carbonaceous polymer, e.g. a fluoropolymer such asptfe, optionally itself a film, which may be porous or microporous. Theprocess can also be applied to other polymers such as polyethylene and arange of other materials used for the biocompatible properties conferredby the acidic groups. Additionally by conversion to functionalitiesterminating in perfluoroalkyl groups the superhydrophobic properties ofthe closely spaced CF₃ groups can be utilised. In certain applicationsit is commercially attractive to change the surface properties of lowcost materials such that they become superhydrophobic. For examplecellulose or polyurethane foam are used for their absorbent nature inwound dressings and incontinence and other sanitary products. By virtueof the hydrophobic layer being present the wicking effect can bedirected and the flow of exudate or moisture constrained. Similarly forfluids with lower surface tension a superhydrophobic or oleophobic layerwould offer the same mechanism.

[0018] A specific embodiment of the invention will now be described byway of example with reference to the accompanying drawings (all graphs),in which:

[0019]FIG. 1 shows C(Is) XPS peak fit for 2 W continuous wave plasmapolymer of acrylic acid.

[0020]FIG. 2 shows continuous wave plasma polymerisation of acrylic acidas a function of power: (a) Q1s) XPS spectra; and (b) O/C ratio andpercentage retention of acid functionality.

[0021]FIG. 3 shows C(Is) XPS spectra for electrically pulsed plasmapolymerisation of acrylic acid: (a) as a function of T_(on) (T_(off)=4ms and P_(p)=5 W); and (b) as a function of T_(off) (T_(on)=175 μs andP_(p)=5 W).

[0022]FIG. 4 shows dependence on average power of: (a) oxygen:carbonratios; and (b) percentage acid group incorporation for continuous wave;and electrically pulsed plasma polymerisation of acrylic acid as afunction of T_(on) (T_(off)=4 ms and P_(p)=5 W and 70 W) and T_(off)(T_(on)=175 μs and P_(p)=5 W).

[0023]FIG. 5 shows variation in the O/C ratio and percentage acid groupincorporation during electrical and gas pulsed plasma polymerisation ofacrylic acid using different gases (T_(on)=175 μs T_(off)=4 ms andP_(p)=5 W).

[0024]FIG. 6 shows electrical and gas pulsed plasma polymerisation ofacrylic acid using oxygen as a function of T_(on) (T_(off)=4 ms andP_(p)=5 W): (a) C(Is) XPS spectra; and (b) O/C ratio and percentage acidgroup retention.

[0025]FIG. 7 shows 2 W continuous wave plasma polymerisation of acrylicacid as a function of oxygen pressure: (a) C(Is) XPS spectra; and (b)O/C ratio and percentage retention of acid functionality.

[0026]FIG. 8 shows electrical and gas pulsed plasma polymerisation ofacrylic acid with oxygen as a function of T_(off) (T_(on)=175 μs andP_(p)=5 W): (a) C(Is) XPS spectra; and (b) O/C ratio and percentage acidgroup retention.

[0027]FIG. 9 shows ATR-IR spectra of: (a) acrylic acid monomer; and (b)Electrical and gas pulsed plasma polymer of acrylic acid, using oxygen,deposited on polyethylene (T_(on)=175 μs, T_(off)=4 ms, and P_(p)=5 W),and

[0028]FIG. 10 shows XPS spectra of plasma polymerisation of acrylic acidunder CW, electrically pulsed and electrically-and-gas pulsed plasmaconditions

[0029] All plasma polymerisations were performed in an electrodelesscylindrical glass reactor (50 mm diameter) enclosed in a Faraday cage.The reactor was pumped by a two stage rotary pump (Edwards E2M2) via aliquid nitrogen cold trap (base pressure of 5×10⁻³ mbar). Power wassupplied from a 13.56 MHz source to a copper coil (10 turns) woundaround the plasma chamber via an L-C matching unit and power meter.

[0030] Prior to each experiment, the reactor was scrubbed clean withdetergent, rinsed with isopropyl alcohol, oven dried and further cleanedwith a 50 W air plasma ignited at a pressure of 0.2 mbar for 30 minutes.A glass slide which had been washed in detergent, then ultrasonicallycleaned in 1:1 cyclohexane and IPA for one hour, was positioned at thecentre of the copper coils and the system pumped back down to basepressure.

[0031] Before polymerisation the acrylic acid (Aldrich 99%) was subjectto several freeze thaw cycles and used without further purification. Themonomer vapour was admitted via a needle valve (Edwards LV 1OK) to apressure of 0.2 mbar for 2 minutes prior to ignition of the plasma. Ifgas was also to be added it was introduced via a needle valve (EdwardsLV 1OK) to the required pressure. For gas pulsing experiments, gas waspulsed into the system by a gas pulsing valve (General Valve Corporation91-110-900) driven by a pulse driver (General Valve Corporation IotaOne). Both continuous wave and pulsed plasma polymerisations wereperformed for 10 minutes.

[0032] For pulsed plasma experiments the R.F. generator was modulated bypulses with a 5 V amplitude supplied by the pulse driver used to drivethe gas pulsing valve. Pulse outputs from both the pulse generator andthe R.F. generator were monitored by an oscilloscope (Hitachi V-252).For experiments involving both gas and electrical pulsing the pulsedriver was used to simultaneously supply the gas pulsing valve and theR.F. generator. Thus the gas pulsing valve was open while the plasma wason.

[0033] Upon termination of the plasma, the reactor system was flushedwith monomer and gas (where applicable) for a further 2 minutes, andthen vented to air. Samples were then immediately removed from thereactor and affixed to probe tips using double sided adhesive tape foranalysis.

[0034] A Vacuum Generators ESCA Lab Mk 5 fitted with an unmonochromatedX-ray source (Mg Kα=1253.6 eV) was used for chemical characterisation ofthe deposited films. Ionised core electrons were collected by aconcentric hemispherical analyser (CHA) operating in constant analyserenergy mode (CAE=20 eV). Instrumentally determined sensitivity factorsfor unit stoichiometery were taken asC(1s):0(1s):N0s):Si(2P)=1.00:0.39:0.65:1.00. The absence of any Si(2p)XPS feature following plasma polymerisation was indicative of completecoverage of the glass substrate. A Marquardt minimisation computerprogram was used to fit peaks with a Gaussian shape and equal full widthat half-maximum (FWHM).

[0035] Results

[0036]FIG. 1 shows the C(1s) envelope obtained by XPS analysis ofacrylic acid plasma polymer. Five different carbon functionalities werefitted: C _(x) H_(y) (285 eV), C CO₂ (285.7 eV), C O (286.6 eV), O—CO/C═O (287.9 eV), and CO₂ (289.0 eV). The hydrocarbon peak was used as areference offset. The oxygen:carbon ratio was calculated by dividing theoxygen peak area (after the sensitivity factor had been taken intoaccount) by the carbon peak area. The relative amounts of acidic carbonatom retention was compared by calculating the percentage of CO₂functionality relative to the total C(1s) area. Continuous waveexperiments were carried out at discharge power between 1.5 and 7 W,FIG. 2. As reported in earlier studies greater oxygen incorporation andacid group retention is achieved on decreasing the power of thedischarge. The best results were found at a discharge power of 1.5 Wwhich gave an O/C ratio of 0.52±0.02 and an acid group retention of18%±1.

[0037] This is considerably less than the oxygen:carbon ration of 0.67and an acid group of 33% anticipated from the monomer structure. Variouselectrical pulse plasma polymerisation experiments were investigated inan attempt to improve retention of the monomer structure, FIGS. 3 and 4.It was found that decreasing the average power of a pulse modulatedplasma discharge, by systematically reducing the plasma ontime orincreasing the time-off, enhances oxygen incorporation and acid groupretention in the plasma polymer. Both the oxygen:carbon ratio and thelevel of acid group retention found under the lowest average powerconditions are significantly greater than found for the continuous waveexperiments. The O/C ratio at the lowest average power was found to be0.72±0.03 and the acid group retention was 30%±1.

[0038] Pulsed addition of various gases was found to increase O/Cratios, FIG. 5. The percentage acid group showed less variation exceptwhen the gas used was oxygen. A large increase, well above monomervalues, in both the O/C ratio and acid group retention is evident whenoxygen is added to the plasma.

[0039] Gas and electric pulse time-on greatly influence the plasmapolymer composition, FIG. 6; at gas and electrical pulse on times belowapprox. 130 μs, the electrical power of the plasma is dominant. Theeffect of oxygen in the system is negligible. Decreasing the time-onincreases the functionality of the plasma polymer. Beyond 140 μs theoxygen partial pressure in the system becomes non trivial. Thecomposition of the thin films produced are altered markedly by thisincrease in the partial pressure of oxygen reaching a maximum at approx.175 μs. Under these conditions the oxygen: carbon ratio was 1.00±0.04and the percentage acid group was 43%±2.

[0040] Continuous wave polymerisation in the presence of oxygen has adirect influence on the functionalisation of films formed, FIG. 7.Increasing the oxygen content in a low power continuous wave plasmaincreases the O/C ratio and the percentage acid group retention. theeffect is less pronounced than for pulsed modulated systems.

[0041] Increasing the plasma and gas time-off for the electrical and gaspulsed plasma polymerisation of acrylic acid using oxygen decreases thefunctionalisation of the films produced, FIG. 8. This is the opposite tothe trend reported above for the electrically pulsed polymerisation ofacrylic acid alone and it may be attributed to the decrease in oxygencontent of the plasma with increasing gas time-off.

[0042] The ATRAR spectrum of the acrylic acid monomer has the followingpeaks, FIG. 9a: O—H stretch (3300-2500 cm⁻¹), C—H stretch (2986-2881cm³¹ ¹), C═O stretch (1694 cm⁻¹), C═C stretch (1634 cm⁻¹), O—H bend(1431 cm⁻¹), C—O stretch (1295-1236 cm⁻¹), C—H out-of-plane bend (974cm⁻¹), O—H out-of-plane bend (918 cm⁻¹), and ═CH₂ wagging (816 cm⁻¹). AnATR-IR of the plasma polymer deposited onto polyethylene,

[0043]FIG. 9b, demonstrates a large amount of oxygen functionalisationwith the O—H bend and C═O stretches clearly evident.

[0044] To optimise the derivatisation of the poly(acrylic acid) orsimilar layer with fluorinated surfactant, the reaction between acarboxylic acid (or e.g. ethylene oxide or styrene oxide) and afluorinated amine may be used. The fluorinated surfactant may be forexample

[0045] Dupont FSD™, a commercially available fluorinated surfactant witha terminal CF₃ group, the opposite end possessing a cationic head basedon a substituted ammonium ion, or

[0046] Hoechst AG 3658 ™

[0047] F₃C—(CF₂)_(n)—CH₂—CH₂—N⁺(Alkyl)₃I.

[0048] Fluoroalkyl trialkyl ammonium salt.

[0049] Formation of the sodium salt of the poly(acrylic acid) PAA isfollowed by reaction with a solution of the fluorinated surfactant, thecarboxylate anion and the cationic fluorosurfactant forming a salt withthe fluoro-chain (terminating in a CF₃ group) uppermost. e.g.

[0050] An alternative route involves a further cold plasma step usingsulphur hexafluoride, SF₆. This reagent will yield CF₃ groups whenreacted with carboxylic acids or with esters.

[0051] Double pulsing could be carried out on other plasma polymersystems—for example with fluorinated monomers like perfluorohexane oreven perfluorocyclohexane, to encourage the preferential coating by CF₃rather than CF₂. The pulsing technique allows one polymerisation pathwayto be favoured over another by changing the time on and time off periodsfor the plasma, so influencing the reaction kinetics.

[0052] A very high degree of functional group control has been achievedby the combined pulsing techniques; see FIG. 10.

1. A method of applying a fluoropolymer film to a porous or microporousor other body, comprising exposing the body to cold plasmapolymerisation using a pulsed gas regime to form either (i) an adherentlayer of unsaturated carboxylic (e.g. acrylic) acid polymer on thesurface and then derivatising the polymer to attach a perfluoroalkylgroup terminating in —CF₃ trifluoromethyl, or (ii) a polymer of aperfluorocarbon monomer.
 2. A method of applying a fluoropolymer filmaccording to claim 1 wherein a combination of electrical and gas pulsingis used.
 3. A method of applying a fluoropolymer film according to claim1 or 2 wherein the cold plasma polymerisation uses a perfluorocarbonmonomer or an unsaturated carboxylic acid.
 4. A method of applying afluoropolymer film according to claim 1, 2 or 3 where both the “gas on”and “gas off” times are from 0.1 microsecond to 10 seconds.
 5. A methodaccording to any preceding claim, wherein perfluorocarbon monomer is notused and wherein the pulsed gas used is oxygen.
 6. A method according toclaim 1, 2, 3, or 4 wherein the pulsed gas used is a noble or inert gasor is hydrogen, nitrogen or carbon dioxide.
 7. A method according toclaim 2, 3, or 4 wherein acrylic acid polymer precursor orperfluorocarbon monomer is pulsed directly without a process gas.
 8. Amethod according to any preceding claim wherein the body is notmicroporous.
 9. A method according to any preceding claim, whereinperfluorocarbon monomer is not used and wherein the plasma power appliedis in the range 1 Watt to 100 Watt.
 10. A method according to claim 9,wherein the plasma power applied is 1.5 Watt to 7 Watt.
 11. A bodyhaving a hydrophobic surface obtained by a method according to anypreceding claim.
 12. A body according to claim 11, whose substrate iscarbonaceous, ceramic, metallic or a combination of these.